2. amps-preamps
  3. 1092202479

Douglas Self on Negative feedback and distortion

I've been reading Douglas Self book on amplifier design and something he said that really makes me think twice.

As you have seen most amplifier makers claim that their amps either does not use global NFB at all or very little of it to improve dynamic (or transient response).

According to Self, the only parameter that matters is distortion and nothing else. I supposed he measures the extra harmonics that the amp produces given a sinusoidal input. In other words, distortion is measured in the frequency domain.

If I remember correctly in my Control Theory course way back in my college days, the frequency domain reponse cannot tell how the amp will response for a given step input. And the STEP RESPONSE is what can tell a lot about the behavior of an amp dynamic and transient response.

In his book, he is very adamant about his position that the only thing that matters is the amp frequency response.

I don't thing frequency response contains information about how any amp would respond to a step input but I could be wrong. Frequency response is only a steady state behavior of the amp. It cannot tell how much the amp would over-shoot, under-shoot, tendency to ringing, and so and so, given a step response. I don't think you can look at the frequency response and make any conclusion about the amp tendency to overshoot, undershoot, ringing and so on...

What do you think?

By the way, I think his book is excellent read into the theory an amplifier design if you can ignore some of his more dogmatic position.
andy2

Showing 5 responses by eldartford

eldartford
4,262 posts
 
Step response can tell you everything you need to know about an amplifier, but criteria are not well defined. From what points do you measure rise time? 10 percent to 90 percent of max amplitude? How many overshoots? How big does the overshoot have to be to be counted? Etc. Etc. You can look at a scope trace and say "that's good" or "thats bad", but that is not very definitive.

Frequency response is not the same thing as step function response, but it is correlated. And a lot easier to measure. If an amp is 0.5 dB down at 100 Kc it probably has good step response.

Negative feedback is most often thought of as a way to reduce distortion, and it does do that. However, it also extends bandwidth, most notably in the case of pentode tube amp output transformers, where the feedback is provided by dedicated windings of the transformer that are connected to auxillary grids of the output tubes.
eldartford
4,262 posts
 
Audioengr...You may be clever enough to design "an amp that has bandwidth to 1 MHz, but has lousy step reponse (overdamped). " But why would you, or anyone else, do that. In practice there is correllation.
eldartford
4,262 posts
 
Sean...."there are loop-holes to every generalization" is one.
eldartford
4,262 posts
 
As a test design engineer (now happily retired)I have a different understanding of the purpose of specs.

Performance of the equipment, we all agree, is determined by its design. A prototype is built and its performance is good. Now we want to put it into production. How can we be sure that each unit that comes off the line is as good as the prototype? We certainly cannot do, on each production unit, the exhaustive performance evaluation that was done on the prototype.

The designer identifies the parameters that he believes are critical to his design, and limits (maximum values) are defined for these parameters. Now, when production units are to be evaluated it is straightforward to measure these parameters. It can even be automated.

So, specs are important...they assure that the unit you buy performs like the one that the designer evaluated (and liked well enough to put into production).
eldartford
4,262 posts
 
Andy2...If the specs that describe the good amp were properly chosen, the laws of physics would DICTATE that a circuit having the same specs would perform in the same way. The electrons would not know whose circuit they were traveling through.
More to discover